In a film manufacture method, sequential biaxial stretching is performed which includes stretching a film under manufacture in a Machine Direction (MD) and then stretching the film in a Transverse Direction (TD). The stretching of the film can reduce the coefficient of thermal expansion of the film in the stretching directions.
An example of the film is a polyimide film. Since the polyimide film has various excellent properties such as light weight, flexibility, film strength, and heat resistance, the film is used in a variety of fields, especially in electronics and electrical fields, as the material of a flexible wiring board, the material of a COF substrate, and the like.
In a known method of manufacturing the polyimide film through the stretching, a solution containing a polyimide precursor such as polyamic acid in a solvent is cast onto a support to provide a self-supporting film (also referred to as a gel-like film, a gel film or the like), the self-supporting film is peeled from the support, then the film is pulled in the MD to perform the stretching by taking advantage of a difference in circumferential velocity between two rollers placed at an interval in the MD direction approximately at a heating temperature at which imidization does not proceed, and the self-supporting film stretched in the MD is then stretched in the TD and heated with both ends held by a tenter apparatus or the like (this method is also referred to as thermal cure).
Patent Document 1 has disclosed a polyimide film produced by casting a solution of polyamide acid containing a ring closure catalyst and a dehydrating agent in an organic solvent onto the surface of a support, imidizing the polyamide acid to form a continuous gel film having a self-supporting property and a solid content of 5 to 50 wt %, stretching the gel film in a traveling direction by 1.1 to 1.9 times, and stretching the film in the TD by 0.9 to 1.3 times the stretching factor in the traveling direction.
A conventionally known method of stretching the film in the MD includes pulling the film in the MD to perform the stretching by taking advantage of a difference in circumferential velocity between two rollers placed at an interval in the MD of the film. In the conventional stretching, however, a so-called neck-in phenomenon occurs in which the film is shrunk in the TD during the stretching in the MD.
When the neck-in occurs, the stretching of the film in the TD at the next step requires not only stretching of the shrunk amount of the film but also stretching of a desired stretching amount in the TD, which results in the need to increase the total stretching amount. The increased stretching amount easily causes a film break to reduce the efficiency of manufacture of the film. The increased stretching amount also adds the mechanical load to a stretching apparatus. In addition, the occurrence of the neck-in may lead to nonuniform film properties in the TD.
To address them, Patent Document 2 has disclosed a stretching apparatus which includes a unique heater apparatus having an improved heat amount applied per unit area to supply the heat amount necessary for stretching to a film in a short transfer distance, so that the neck-in amount can be reduced.